1. Field of the Invention
The invention relates to a method for treating a liquid egg product containing egg white and/or egg yolk in order to obtain an extended shelf life, where the egg product is subjected to rapid heating and to a pasteurization treatment, whereby the egg product is maintained at a temperature of more than approximately 60.degree. C. for some time, for example a few minutes.
2. Discussion of the Background
The use of a pasteurization treatment is a generally known method which is, for example, described in U.S. Pat. No. 4,808,425. According to this patent an egg product is subjected to a temperature of 63-72.degree. C. for a period of time ranging from a few seconds to more than three minutes. The egg product is heated to the desired temperature by means of a plate-type heat exchanger, and after having been maintained at that temperature for a sufficiently long period of time, the egg product is cooled down to a temperature below 10.degree. C. and subsequently packed. According to this patent, a more intensive heat treatment will result in coagulation of the egg product, especially the loss of solubility of protein, which amounts to more than 15% and is unacceptable. The loss of functional properties of the egg product, which is still acceptable according to this patent, imposes considerable limitations with regard to the applications of the pasteurized egg product, however.
U.S. Pat. No. 4,957,760 teaches a method of ultrapasteurizing a liquid egg product by contacting the egg product to steam. Preferably, the steam is held at not less than atmospheric pressure and the temperature of the product is not raised more than about 37.degree. F. in this heating step. Following the direct steam heating step, the steam heated product is subjected to a holding time sufficient to cause at least a 9 log cycle reduction of Salmonella bacteria in the product. The holding time is preferably at least 10 seconds long and is taught as reducing the operating problems inherent in processes in which a product is contacted with steam under a vacuum.
U.S. Pat. No. 3,113,872 discloses a method of treating liquid egg products in which liquid egg is first heated by contact to a heated surface and then cooled, followed by a second heating step in which the egg product is contacted to steam followed by a second cooling step. In the steam heating step, the egg is heated to a temperature between 155-165.degree. F. (68.33-73.89.degree. C.) for about 1 second under a vacuum. In the steam heating chamber, a portion of the liquid egg impinges on the walls of the chamber and runs to the bottom of the chamber through the steam atmosphere.
Sugihara et al, "Heat Pasteurization of Liquid Whole Egg", 1966, Food Technology, 20(1) pp. 100-107 discloses heat pasteurizing liquid whole egg in which liquid egg is heated by two temperatures of 150-170.degree. F. (65.56-76.67.degree. C.) under vacuum, for 2-3 seconds. During operation, the liquid egg passes through a plate heating section and then into a cylindrical steam-infusion chamber where the liquid egg is distributed as a thin film along the sides of the chamber where it is heated for 2-3 seconds by steam introduced into the chamber. A similar process is described by Kline et al, "Heat Pasteurization of Raw Liquid Egg White", 1965, Food Technology, 19, pp. 105.
Although known pasteurization treatment will result in an egg product having a reasonable shelf life, dependent on the quality of the basic product and the setting of the treatment plant, there is a need for a method for treating a liquid egg product, which makes it possible, by using a reliable process, to obtain an egg product having an extended shelf life which meets all quality requirements and whose functional properties have hardly diminished, it at all.
Further, conventional pasteurization of egg products is known to kill most bacteria. However, some bacteria are particularly resistant to pasteurization under conventional conditions, for example Bacillus cereus. A need continues to exist for heat treatment procedures with improved bacteria killing properties.
A continuing problem in the treatment of liquid egg products is the necessity to pasteurize the egg product at a sufficiently high temperature to effect pasteurization and yet at a temperature low enough to prevent loss of physical and functional properties of the egg product. This problem is compounded by the necessity to kill a wide range of bacteria which may be present in the egg product and, specifically, bacteria which have demonstrated a resistance to pasteurization. Processing conditions which are insufficient to adequately pasteurize the egg product and achieve sufficient bacterial killing result in a packaged product with an unsatisfactorily short shelf life. The present invention solves the problem noted above by providing processing conditions which pasteurize the egg product in a way which maintains the functional and physical properties of the egg product and achieves high bacterial killing to provide a packaged egg product with long shelf life.